Аннотация:Optical monitoring strategies are especially important for reliable control of deposition processes because they allow one to monitor optical thicknesses of coating layers, which are the most important production parameters. The chapter discusses broadband and monochromatic monitoring strategies, direct and indirect monitoring approaches, and passive and active monitoring strategies. In the case of monochromatic monitoring, special attention is given to turning point, level monitoring, and monitoring by swing. The most essential effects connected with optical monitoring, such as error self-compensation effect and cumulative effect of thickness errors, are considered.